An insulating liquid for use in electrical distribution and power equipment, including transformers, has two important functions. First, it acts as an electrical insulating medium and, second, it transports heat generated in the equipment. For example, heat is transported from the windings and core of the transformer or connected circuits to cooling surfaces. In addition to possessing the dielectric strength and cooling capacity, the ideal insulating liquid should be environmentally compatible and relatively nonflammable.
For over a century, mineral oils derived from crude petroleum have been used extensively as insulating and cooling liquids in electrical equipment. However, as safety standards became more demanding for many indoor and vault equipment installations, such oils were replaced to a great extent by non-flammable liquids, such as askarel (polychlorinated biphenyl-PCB) fluids. Beginning in the 1930's, PCB's, which are generally considered to be nonflammable, were widely utilized as replacements for mineral oils as insulating liquids in electrical equipment. Nonflammability is a required property for insulating oils that are used in equipment placed within or around building structures, as it is necessary to minimize the hazard of fire and explosion damage in the event of electrical faults within the equipment.
Eventually, it became recognized that PCB's are environmentally hazardous liquids. As a result, the production and sale of PCB's and their use in new equipment was banned. For existing PCB-containing equipment, severe regulations were issued requiring PCB removal at certain installations and severe restrictions for all other installations. In addition, spill reporting, clean-up and disposal require compliance with very strict regulations outlined in U.S. EPA rules published in various editions of the Federal Register. Furthermore, due to their relatively poor ability to suppress arcs and harmful arc-degradation by-products, PCB-based fluids were not applied to immersed safety and operational devices such as submerged power fuses, circuit breakers, and load-break switches.
Because of the disadvantages and shortcomings of the polychlorinated biphenyls, there have been numerous efforts made to develop relatively inexpensive, environmentally safe, nonflammable insulating oils. To date these efforts have not been completely successful. It is the general object of the present invention to provide electrical equipment utilizing an insulating liquid that is non-toxic, biodegradable, relatively nonflammable, innocuous to the environment, and comparatively inexpensive. In addition, the insulating oils typically conform to existing specifications or guides for dielectric fluids and must exhibit performance characteristics that are generally comparable to presently used insulating oils.
Some of the functional properties of the oil and their significance are as follows. An oil's dielectric breakdown at 60 Hertz indicates its ability to resist electrical breakdown at power frequency and is measured as the minimum voltage required to cause arcing between two electrodes submerged in the oil. The impulse dielectric breakdown voltage indicates its ability to resist electrical breakdown under transient voltage stresses such as lightning and power surges. The dissipation factor of an oil is a measure of the dielectric losses in that oil. A low dissipation factor indicates low dielectric losses and a low concentration of soluble, polar contaminants. The gassing tendency of an oil measures its tendency to evolve or absorb gas under conditions where partial discharge is present.
Because one function of the dielectric fluid is to carry heat, factors that significantly affect the relative ability of the fluid to function as a dielectric coolant are viscosity, specific heat, thermal conductivity, and coefficient of expansion. The values of these properties, particularly in the range of operating temperatures for the equipment at full rating, are weighed in the selection of suitable dielectric fluids.
In addition to all of the foregoing properties that affect heat transfer, a dielectric fluid for commercial use should have a relatively high dielectric strength, low dissipation factor, a dielectric constant compatible with the solid dielectric, a low gassing tendency, and must be compatible with typical electrical equipment materials that are exposed to it. In order to function properly, the material must have an adequate heat transfer capability, which depends on its viscosity, specific heat and coefficient of expansion.
Current codes and standards further require that any dielectric fluid intended for use as a coolant must not be classified as Flammable, but rather as a Class IIIB Combustible liquid. The safety requirements depend on the application in which the electrical equipment containing the fluid will be used, such as indoor, rooftop, vault, and adjacent to building installations. According to the degree of hazard, one or more safeguards may be required. One recognized safeguard option is the substitution of conventional mineral oil with Less-flammable and Non-flammable liquids. Less-flammable liquids must have an open-cup fire point equal or greater than 300.degree. C.
As described above, several operable fluids are known and used in electrical equipment. However, due to increasing awareness and sensitivity regarding environmental concerns, it has become desirable to provide a dielectric fluid that has minimal effect on the environment and degrades quickly and easily enough so that spills will not contaminate the soil or the water table for any significant period of time, nor represent a significant hazard prior to the natural biodegradation process. It is becoming more desirable to replace non-renewable resources with renewable resources, particularly in the area of petroleum based products. There is increased demand by purchasers for all-natural products. Finally, more attention is being placed on the long-term effects of materials and their degradation by-products. All these environmental, health, and safety trends favor the use of vegetable based dielectric coolants over those derived from petroleum.
The oils derived from various plants, herein referred to as "vegetable oils," include many oils that have suitable dielectric properties when the oil is fresh and carefully processed. It is often the case, however, that such oils are particularly susceptible to polymerization when exposed to free oxygen. The rate of polymerization is directly related to the temperature of the oils at the time of exposure to free oxygen. Exposure to oxygen activates unsaturated bonds, causing oxidative polymerization of the oil, with potentially adverse effects on both equipment in the fluid and on the properties of the fluid itself.
Many types of electrical power distribution equipment, including transformers, are low-maintenance equipment that may go many years without inspection. The presently used mineral oils are significantly less susceptible to degradation due to exposure to oxygen than vegetable oils and therefore typically pass the standard oxidation stability tests. Therefore, mineral oils are well suited to use in this type of electrical equipment due to their long operable life. Correspondingly, until now there has been no acceptable way to effectively reduce the long-term effects of exposure of vegetable oils to oxygen, so vegetable oils have not been successfully used as dielectric coolants in modern electrical equipment. It is therefore desired to provide a low maintenance vegetable oil based dielectric coolant that meets or exceeds safety standards and is environmentally innocuous.
These and other objects and advantages of the invention will appear from the following description.